1. Field of the Invention
The present invention relates to a wireless communication system and method using multiple antennas. More particularly, the present invention relates to a wireless communication system and method using multiple antennas, by which a transmission throughput can be maximized in a multi-user environment employing a high-speed downlink packet access (HSDPA).
2. Description of the Related Art
Unlike conventional personal communication service (PCS) wireless communication systems, next-generation wireless communication systems transmit data at a higher rate. To transmit packets at a high speed through a downlink, the 3rd Generation Partnership Project (3GPP), a standardization organization for asynchronous mode, led by Europe and Japan, has standardized an HSDPA technique, and the 3GPP2, a standardization organization for synchronous mode, led by the United States, has standardized a 1xEvolution Data Only/Voice (1xEV-DO/V) technique.
The two standard organizations have suggested HSDPA to smoothly provide Internet services including web services. The HSDPA technique is optimized to a peak throughput as well as an average throughput, thereby accomplishing smooth transmission of packets, such as data, as well as circuit transmission, such as voice communication.
To implement the HSDPA technique, an Adaptive Modulation and Coding (AMC) technique, a Hybrid Automatic Repeat reQuest (HARQ) technique, and a multi-user diversity scheduling technique are fundamentally needed. Furthermore, to overcome a limit of a given bandwidth, an efficient multiple antenna technique is required. These techniques are described on the Internet site of the 3GPP, www.3gpp.org, and described in detail in an official document TR25.858, which may be located at http://www.3gpp.org/ftp/specs/html%2Dinfo/25858.htm. The official document TR 25.858 is entitled “Physical Layer Aspects of UTRA (UMTS Radio Access Network) High Speed Downlink Packet Access,” reported by Amitabha Ghosh.
In multi-user diversity scheduling, channels through which users request packets are fed back, a mobile station on a channel in a best state among the fed back channels is detected, and the packets are preferentially transmitted to the detected mobile station so that a diversity effect including signal-to-noise ratio (SNR) gain can be achieved. A diversity order indicating a degree of diversity gain corresponds to a number of users requesting packets simultaneously.
The multiple antenna technique overcoming the limit of a bandwidth uses a geometric space axis when it is used by way of beam-forming, and therefore, the limit of resources of bandwidth in a frequency axis can be overcome. To increase the bandwidth in the frequency axis, a nulling technique is used. Beam-forming is a process that performs nulling using a correlation between antennas which are close to each other, for example, separated from each other by a distance of λ/2 (where λ is a wavelength). Nulling is a technique of conditioning antenna weights to satisfy w1Hh2=0 and w2Hh1=0 so that a signal r1 received by a first user does not include data d2 of a second user while a signal r2 received by the second user does not include data d1 of the first user.r1=(d1w1H+d2w2H)h1+n1=(d1w1H+0)h1+n1r2=(d1w1H+d2w2H)h2+n2=(0+d2w2H)h2+n2  Formula (1)
Here, w1 and w2 denote matrices expressing weights, respectively, h1 and h2 denote channels, n1 and n2 denote noises mixed with the respective reception signals r1 and r2, and H indicates a Hermitian matrix.
When channel conditions are configured to produce weights satisfying Formula (1), influence on other user channels can be completely removed, so that substantial transmission capacity can be doubled. Theoretically, if the number of other users to be nulled in a beam-forming environment is one fewer than the sum of the number of users requesting signal transmission and the number of antennas, nulling is always possible. However, such an idealistic state can be accomplished when correlation between antennas completely exists and only phases of the antennas differ from each other. Accordingly, it is very difficult to implement a beam-forming nulling technique in a wireless communication environment. Moreover, in diversity systems including multiple antennas to overcome channel fading, since antennas are far apart from each other by, for example, 10 λ, correlation between antennas rarely exists. Consequently, it is difficult to adopt a usual nulling technique applicable to beam-forming to diversity systems.
When there are multiple users wishing to transmit data to a base station simultaneously, usually multi-user transmission using orthogonality of a spreading code is used. However, even in a method using the orthogonality of a spreading code, multipath fading occurs, resulting in self interference (SI) between multiple codes and multiple access interference. As a result, a nulling effect due to a spreading code is reduced, and therefore, performance is remarkably degraded. Accordingly, the nulling technique is required to be applied to multiple antennas in diversity mode.
The following description concerns a multiple antenna technique in diversity mode.
Typically, wireless communication systems are configured so that multiple mobile stations communicate with one another through a single base station. To transmit data at a high speed in such wireless communication systems, loss according to a characteristic of a wireless communication channel, such as fading, and interference for each user need to be minimized. Fading may reduce the amplitude of a reception signal by several dB through several tens of dB. To overcome fading, a variety of diversity techniques may be used.
Code Division Multiple Access (CDMA) usually uses a rake receiver performing diversity reception using delay spread of a channel. The rake receiver employs a reception diversity technique for receiving multipath signals. However, such a diversity technique has a problem in that it does not operation when a delay spread is small.
Diversity is largely divided into space diversity and time diversity. Space diversity uses multiple antennas. Time diversity uses interference and coding and is usually used for a Doppler spread channel. However, it is difficult to use time diversity in a low-speed Doppler channel. Generally, space diversity is used to overcome fading in an indoor channel having a small delay spread and in a pedestrian channel, which is a low-speed Doppler channel. Space diversity uses two or more antennas so that when a signal received through one antenna is attenuated due to fading, the signal can be received through another antenna. Space diversity is divided into transmission diversity and reception diversity according to an employed antenna type. It is difficult to install reception antennal diversity in a mobile station in terms of installation area and cost, and therefore, it is recommended to use transmission antenna diversity in a base station.
Transmission antenna diversity is divided into closed loop transmission diversity, in which a base station receives downlink channel information fed back from a mobile station, and open loop transmission diversity, in which there is no feedback from a mobile station to a base station. In transmission diversity, a mobile station measures the phase and amplitude of a channel and finds an optimal weight. A base station sends different pilot signals through respective antennas to measure the amplitude and phase of a channel. The mobile station measures the amplitude and phase of the channel using a pilot signal and finds an optimal weight based on the measured amplitude and phase of the channel.
In multiple antenna systems using diversity, transmission antenna array (TxAA) mode 1 and TxAA mode 2 are closed loop transmission diversity using two antennas. TxAA mode 1 and TxAA mode 2 were standardized by 3GPP, which is an International Mobile Telecommunications (IMT-2000) standardization organization led by Europe and Japan, and distributed as version Release (R) 99. TxAA mode 1, suggested by Nokia, feeds back only a phase difference between two antennas. TxAA mode 2, suggested by Motorola, feeds back the phases and gains of two antennas. TxAA mode 1 and mode 2 are disclosed in a specification defined by 3GPP, which is the standardization organization for Universal Mobile Telecommunications System (UMTS), which is the standard of European IMT-2000.
In TxAA mode 1 or mode 2 using closed loop transmission diversity, adaptive array antennas are used, and different complex weights are applied to respective antennas. A complex weight is a value related to a transmission channel. For example, w=h is used as the complex weight, where “w” is a transmission array antenna weight vector, and “h” is a vector expressing a transmission array channel.
In wireless communication systems using Frequency Division Duplex (FDD), the characteristics of a transmission channel are different from those of a reception channel. Accordingly, in order to detect a transmission channel in a base station, transmission channel information needs to be fed back from a mobile station to the base station. For this feedback operation, TxAA mode 1 and mode 2 are designed such that a mobile station sends weight information to a base station, and the base station obtains channel information from the weight information. In TxAA mode 1, in information on a weight w=[|w1|exp(jθ1),|w2|exp(jθ2)] multiplied to each antenna, only a phase difference θ1-θ2 is quantized in two bits and fed back. Accordingly, a phase accuracy is π/2, and a maximum quantization error is π/4. To increase feedback efficiency, refining is performed by updating one of two bits in each feedback. For example, two bits can be combined like {b(2k), b(2k−1)} and {b(2k), b(2k+1)}. Here, “b” is a bit that is fed back in each slot, and 2k, 2k−1, and 2k+1 denote a feedback sequence of bits. In TxAA mode 2, both phase and gain that constitute weight information are fed back. A phase is fed back in three bits, and a gain is fed back in one bit. Accordingly, a phase accuracy is π/4, and a maximum quantization error is π/8. To increase feedback efficiency, progressive refining is performed by updating one among four bits in each feedback. In a refining method, each bit becomes values of an orthogonal basis. However, in a progressive refining method, such a rule is not defined.
Systems using diversity are harmonized with the characteristics of a space channel to enable coherent transmission. Consequently, in proportion to the number of antennas, an SNR increases and fading can be overcome. When a nulling technique is additionally used, a transmission throughput can be increased.
However, the nulling technique can be applied only to a beam-forming method in which a distance between antennas is limited to a predetermined value. Therefore, it is difficult to use the nulling technique for multiple diversity antennas far apart from each other. Accordingly, multiple diversity antenna systems for voice transmission are structured to omit the nulling technique. In other words, when a distance between antennas increases, a correlation between the antennas decreases. As a result, a channel change period is in inverse proportion to a maximum Doppler frequency. In an existing voice communication environment, a channel used by a user during a single frame duration is not fixed but varies, and therefore, it is impossible to apply nulling to a fixed channel. Moreover, in CDMA, the number of antennas becomes far more than the number of simultaneous users and then exceeds the degree of freedom in nulling multiple antennas, i.e., (the number of antennas−1). Accordingly, it is difficult to apply the nulling technique to a conventional diversity method using multiple antennas.
Therefore, a wireless communication system including multiple transmission/reception antennas, which is compatible with TxAA mode 1 and mode 2 and which can overcome a frame size problem and a problem related to a difference between the number of antennas and the number of users, is desired.